1. Field of the Invention
The invention relates to energy seals for microwave oven apparatus.
2. Description of the Invention
Microwave ovens are widely used in the preparation of products for home and industry due to the rapid heating times resulting from the so-called dielectric heating phenomenon. The energy source typically employed for such apparatus is the magnetron of the type described in the text "Microwave Magnetrons", Vol. 6, Radiation Laboratory Series, by G. B. Collins, McGraw-Hill Book Company, Inc., 1948. The high frequency radiated within an oven enclosure is uniformly distributed by such means as mode stirrers to heat the load by molecular friction resulting from the absorbed energy oscillations. In such apparatus the enclosure dimensions are substantially greater than a free space wavelength of the operating frequency. The allocated frequencies for such apparatus are assigned by regulatory bodies such as the Federal Communication Commission and may be either 915 .+-. 13 MHz or 2450 .+-. 50 MHz in the industrial, scientific and medical band of the electromagnetic energy spectrum. The term "microwaves" is intended to define electromagnetic energy radiation in that portion of the spectrum having wavelengths in the order of 1 meter to 1 millimeter and frequencies in excess of 300 MHz.
Safety standards have been established by regulatory agencies such as the Department of Health, Education and Welfare and the American National Standards Institute for the control of the amount of energy leakage around the door enclosing the access opening to the oven enclosure. In the prior art numerous energy seals have evolved including metal-to-metal contacting arrangements shown, for example, in such U.S. Pat. Nos. as 2,956,143 issued to L. H. Shall, 2,958,754 issued to D. H. Hahn and 3,448,232 issued to J. Kluck. Other types of prior art energy seals include reentrant electrical choke arrangements having dielectric bodies disposed within a choke cavity to provide paths of least resistance for energy leaking along the peripheral gap defined between the door and access opening walls. Examples of the reentrant choke-type energy seals include U.S. Pat. Nos. 3,182,169 issued to R. Ironfield, 3,584,177 issued to A. Bucksbaum and the slotted-type energy seal embodying the teachings of 3,767,884 issued to J. Osepchuk et al.
The metal-to-metal type energy seals are subject to loss of effectiveness due to mechanical variations after long periods of use and problems of electrical arcing when spaces develop between the contacting metal surfaces. The choke-type arrangements are perhaps the most effective since the dimensions are selected to be frequency responsive to result in the reflection of a low impedance defined by a terminating wall at the point of origin of the escaping energy approximately one-half a wavelength away. Continual testing and experimentation with energy seals for microwave ovens has been required to improve such structures and to meet even higher safety standards. One area that has always been of concern is the difficulty in controlling the dimensions of choke-type seals particularly since close mechanical tolerances must be maintained during production. Variances in the dielectric bodies within the choke cavity as well as the spacings between the choke cavity and oven access opening walls defining a peripheral gap have been observed to lead to energy leakage on some sides at a higher level than the remaining sides of overall peripheral gap. As an additional safety factor, therefore, microwave ovens are provided adjacent the outer edge of the door with secondary energy absorbing structures of a plastic material loaded with carbon, graphite, ferrites or other lossy materials.
In addition to the enumerated prior art patents, numerous additional embodiments have evolved in the microwave heating art dealing with additional structures for preventing energy from escaping from the oven enclosure. Examples of such art include U.S. Pat. No. 3,668,357 issued to K. Kobayashi which discloses a second microwave cavity to prevent passage of the second harmonic frequency component of the energy, U.S. Pat. No. 3,544,751 issued to Valles which discloses projecting peripheral access opening walls which extend within channels provided in the door panel to provide a spacing of less than one-tenth of a wavelength of the operating frequency, and U.S. Pat. No. 3,249,731 issued to S. Johnson which provides a tandem arrangement of two quarter wave chokes at the microwave operating frequency.
With the desire for maintaining higher safety standards, it is necessary to provide new and improved energy seals which will cope with the dimensional problems, bearing in mind the need for simplicity and low cost.